The wrong 'PhreeqPython' method is called in the README.md file

[scharlton2]

In the 'Features' section of the README.md file the wrong 'PhreeqPython' method is called:

...
# add a solution consisting of 1 mmol CaCl2 and 2 mmol NaHCO3
solution = pp.add_solution({'CaCl2':1.0,'NaHCO3':2.0})
...

should be:

...
# add a solution consisting of 1 mmol CaCl2 and 2 mmol NaHCO3
solution = pp.add_solution_simple({'CaCl2':1.0,'NaHCO3':2.0})
...

and

solution2 = pp.add_solution({'KCl':1.0})
# create mixture of 50% solution and 50% solution2
...

should be:

solution2 = pp.add_solution_simple({'KCl':1.0})
# create mixture of 50% solution and 50% solution2
...